Ok this is just my take, and by no means am I a rule lawyer. I believe the Dwarven trait allows you to not take movement penalties when wearing heavy or medium armor or Encumbered. but I do not believe this gives you the ability to ignore medium and heavy armor restrictions to skills or class abilities. So the Tumble skill says it cannot be used when wearing medium or heavy armor, so even though the dwarves movement speed is not affected by his wearing of med/hev armor his tumbling ability is (thus the OP ability to tumble in his chain shirt but not plate even though his movement is unchanged). So I think this rule is being applied correctly. Just as the ranger special combat style someone choses at 2nd level it specifically mentions that the style is not useable in med/hev armor. Though a Dwarf ranger can wear plate and still move 20ft but he would loose his rapid shot ability. At least that's how I interrupt it.